The present invention relates to a frictional joining method used for joining different kinds of metal members, and a frictional joining structure formed by the frictional joining method.
Conventionally, a frictional joining method is known as described in Japanese Laid-Open Patent Publication No. 2001-314982, in which the same kind of metal members are lapped, a rotating tool is placed on one of the same kind of metal members and a rotational force and a pressing force are applied to the rotating tool in such a manner that the rotational force is applied around a pressing axis of the rotating tool and the pressing force is applied toward the other of the metal members which is joined to the one of the metal members on which the rotating tool is placed. In this method, frictional heat is generated between the rotating tool and the metal members by pressing and rotation of the rotating toll, and the metal members are softened by the frictional heat, whereby the rotating tool is inserted into the softened metal members. Thereby, the rotating tool induces plastic flow in the metal members around a lapping face of the metal members, and consequently the lapped metal members are joined integrally at a joint spot of the lapping face. However, in a case where different kinds of metal members which are made of hard materials, such as steel, are applied, it is difficult for the rotating toll to induce such plastic flow in the metal members. As a result, enough joining strength can not obtained.
In light of this problem, a different joining method has been proposed recently as described in Japanese Laid-Open Patent Publication No. 2002-66759, in which an alloy is provided between the different kinds of metal members, and both of the metal members and the alloy are diffused into each other by the frictional heat and the plastic flow generated by the rotation of the rotating tool. Thereby, the different kinds of metal members are joined integrally by the use of the alloy.
However, although the latter joining method shows some improved joining strength compared to the former joining method, it is still not enough. Because, a material strength of the alloy itself is not so high, and therefore the joining strength of the metal members with the alloy may not improve sufficiently in total.